APPLICATION OF OVERBAND MAGNETIC SEPARATOR
The first type of magnetic separation equipment is the overhead magnet. These are stationary magnets with self-cleaning belts that rotate around the magnet assembly. The cleated belt moves the attracted ferrous material and sorts it out of the magnetic field. These magnets can be configured in two main ways – parallel to the conveyor, referred to as inline; or perpendicular to the conveyer, referred to as crossbelt. Other configurations are actually variants of the overhead magnet where multiple magnets are used to transfer ferrous material from one magnet to another. These magnets are referred to as "multi-stage" magnets.
In an inline application the magnet is normally positioned at the end of the conveyor above the head pulley. The main advantage to positioning the magnet in this fashion is that entrapment of ferrous pieces and particles is reduced. Material is freed once it leaves the conveyor belt and the magnet can pluck suspended ferrous material out of the air.
If the conveyor is on an incline, the momentum of the particles leaving the conveyor belt results in an initial trajectory upward and toward the magnet. Thus, the material gets closer to the magnet and the ferrous particles have a better chance of getting picked up.
"No matter how hard a processor tries to prevent entrapment, it is always going to occur with an overhead magnet," says one manufacturer of magnetic separation equipment. "But it is not going to occur as much in an inline configuration as it is with in a crossbelt arrangement."
It is especially tough to pull out ferrous from wet, shredded wood streams with an overhead magnet, because the shreds start to interlock and clump. Suppliers say that wet wood and any other wet material is more difficult to process, and should be avoided if possible when applying magnetic separation. However, an inline configuration can free up more of the ferrous material for separation.
For inline applications, the magnet should be the width of the conveyor. Some manufacturers have square magnets. Others offer rectangular magnets where the longer length of the magnet is parallel to the conveyer, providing more coverage of the belt.
The other application for an overhead magnet is in the crossbelt configuration. This is a popular installation because placing the magnet inline over the head pulley is not always practical – there may be other equipment, such as a magnetic pulley or an eddy current separator, at the end of the conveyor. Plus, material recovery facility operators like the crossbelt configuration because the magnet can be positioned close to the hand picking stations, and because slower belt speeds increase the magnet’s efficiency.
In both the inline and the crossbelt configurations, the overhead magnet is working against gravity, so it has to work harder and normally has to be more powerful than a magnetic pulley or drum. However, the inline setup requires less field strength than the crossbelt, because it does not have to combat entrapment, nor does it have to change the direction of the ferrous material. Therefore, an inline overhead magnet can cost less than one used in a cross-belt configuration.
While many manufacturers sell both Permanent Magnetic Overband Separator and Electromagnetic Overband Separator configurations, one manufacturer recommends that a processor use an electromagnet in the overhead position when the distance between the magnet and conveyor has to be greater than 12 inches.
Most overhead applications are with permanent magnets, but processors also like the electromagnet because they can adjust the strength and even shut it down when they don’t need to separate ferrous.